class SomeClass {
    constructor(public somePropOne: string, public somePropTwo: string, public somePropThree?: string) {
    }
}

const sc: SomeClass = new SomeClass("1", "2")
console.log(sc.somePropOne) // "1"
console.log(sc.somePropTwo) // "2"
console.log(sc.somePropThree) // undefined